This application claims priority to a prior Great Britain patent application, No. 0002703.7, filed Feb. 8, 2000.
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This invention relates to a mooring and flowline system for use in floating facilities for handling petroleum and petroleum products. The system is particularly, but not exclusively, applicable to the mooring and loading of floating production ship shaped vessels.
Production from offshore wells can be exported ashore by subsea pipeline, but this requires a very large capital expenditure. More recently there has been a move to exploiting more marginal fields by the use of a floating production, storage and offtake vessel (FPSO), from which oil is exported periodically by tanker. Commonly, the floating production facility has been provided by conversion of an existing tanker. Such arrangements have worked well, but there is a continuing need for a substantial reduction in installation costs in order to improve the economics of marginal field development and production.
Tanker conversions used hitherto have required extensive conversion. In some cases, a turret mooring is used which includes a rotary oil flowline joint, and this requires major structural work on the tanker in addition to the complex mooring turret. In other cases, a flexible riser to the tanker has been used, but has required large quick disconnect (QDC) valves with a physical size and weight requiring installation outboard of the tanker bow on a specially installed and relatively large structure.
An object of the present invention is to provide an improved mooring and flowline system which is simple and economical to install. It is also an object of the invention to provide a system which enables conventional tankers to be used as floating production, storage and offtake vessels with a minimum of structural alteration.
Accordingly, the present invention provides a mooring and flowline system comprising: means for mooring a floating production and/or storage vessel to the seabed, and one or more flowlines for connection to one or more producing wells or to a facility; the mooring means comprising at least two anchors in the sea bed, a respective anchor riser extending from each of the anchors, each of the anchor risers having one end secured to its anchor and the other end secured to a common mooring node member, and mooring pendant means extending from the node member for connection, in use, to the vessel; each flowline comprising a seabed flowline extending from the direction of the well or facility into the vicinity of the mooring means, and a flexible riser pipe extending from the seabed flowline to the vessel; and in which each flexible riser pipe is a continuous conduit with a single axial rotational coupling and has part of its length secured alongside the mooring pendant means.
Preferably, there are at least three anchors in an angular array, three anchors at mutual 120xc2x0 spacings being particularly preferred. The riser pipe is suitably held clear of the anchor risers and node member by providing a portion of the riser pipe to be located through a series of guidance collars, these being attached to the mooring chain pendant to control sideways and rotational movement.
Preferably also, there is no mooring swivel between the anchors and the vessel; the node member may be a plain ring, shackle or plate structure.
The mooring pendant may comprise two parallel chains, one passing over each bow of the vessel; alternatively the mooring pendant may consist of a single chain.
Preferably, the vessel is a tanker.
In a particularly preferred form the invention, which allows a conventional tanker to be utilized as the production vessel with a minimal amount of conversion, the single or twin chains are brought inboard of the tanker to standard chain stoppers or to remotely controlled release stoppers, the riser pipe is brought inboard of the tanker over a chute to have its inboard end lying near the tanker deck, and quick disconnect valve means are located near the tanker deck between said riser pipe and a tank manifold of the tanker.